This project will apply the method of electron probe microanalysis to studies of renal medullary function, in particular, the renal concentrating mechanism and the role of the collecting duct in the regulation of sodium balance. The operation of the concentrating mechanism has remained obscure, despite much effort devoted to measurement and modelling. This work has been hampered by the fact that most of the relevant structures are inaccessible to micropuncture. The electron probe analyzer, which combines high sensitivity and a micron spatial resolution, may make possible the direct analysis of elemental composition in and along collecting ducts and other structures visualized in frozen sections. By combining electron probe analysis with appropriate pre-incubation steps, the activity of Na-K-ATPase in medullary structures will be measured as a function of physiologic states. By means of a new and unique "micro punch" technique, the frozen lumen content of collecting ducts deep in the kidney will be sampled for electron probe analysis of both inorganic solutes and urea.